1. Field of the Invention
The invention in general relates to sensor systems wherein an array of active elements are mounted on a carrier vehicle, and more particularly to apparatus used in a displaced phase center correction system.
2. Description of Related Art
In a side looking sonar system, a transducer on a carrier vehicle periodically transmits pulses of acoustic energy, laterally of vehicle travel, toward a target area such as the bottom of a body of water. Acoustic energy reflected back from the area impinged by the transmitted pulse, is received by a receiver transducer array on the carrier.
The beam pattern associated with the receiver transducer array is relatively narrow in the direction of carrier travel such that it detects reflected acoustic energy from a relatively narrow strip. The received acoustic energy is processed and may be displayed on a suitable display, or stored for future display.
With each pulse transmission and subsequent reception, a scan line is produced on the display and portrayed as a line by line image that is a pattern of highlights and shadows analogous to an optically viewed panorama illuminated by side-lighting, with objects outlined in such a way as to permit their identification.
It may be shown that increasing the length of the receiver array decreases the receive beam width, thus increasing the along-track resolution and allowing for better identification of certain targets. In a synthetic aperture sonar system a relatively narrow receiver beam with very fine angular resolution may be formed by a relatively short transducer array. This is accomplished by periodically transmitting acoustic pulses (pings) during course of travel over the target area and storing acoustic returns provided by the active elements of the transducer array. After a predetermined number of stored return signals are collected, they are suitably processed to form a single beam with a higher resolution than is possible with the original receiver array.
In such systems in order to achieve accurate results, the carrier vehicle upon which the active elements are mounted must not deviate more than a small fraction of a wavelength (of the acoustic signal) from its defined course. Since this may be impractical, systems exist which determine carrier deviations and correct the processed signals to compensated for such deviations.
One such system is an electronic system which operates on a DPC (displaced phase center) algorithm, such as described in U.S. Pat. No. 4,244,036, hereby incorporated by reference. Basically, by virtue of the transmitter transducer and plurality of receiver active elements, fictitious phase centers are established halfway between the transmitter and each receiver active element. Selected ones of these fictitious phase centers, or equivalent spatial sample locations should overlap from ping to ping in the absence of carrier yaw or lateral movement, called sway. In the presence of yaw (rotation about a vertical axis) and/or sway (lateral movement), deviations in time of arrival of the acoustic return at these selected spatial locations, from ping to ping, are utilized to provide an indication of carrier deviation from its course line, such that suitable correction may be made.
In the typical ocean environment, characterized by high noise and multipath reverberation however, the time of arrival estimation can be corrupted. This is in direct odds with long range high area coverage goals where stringent time delay estimation requirements are needed to estimate yaw motions, particularly for long range applications.
The present invention reduces the effect of noise and interference particularly on yaw estimations.